1. Field of the Invention
The present invention relates to a vibration type driving device in which high frequency voltage is applied to a piezoelectric element to generate high frequency vibrations, which are used to move an article.
2. Description of the Prior Art
One known art is so called a Langevin type oscillator including several annular piezoelectric elements interposed between a pair of blocks. An example of such an oscillator is disclosed in U.S. Pat. No. 5,122,700. FIG. 15 shows the Langevin type oscillator which includes a pair of blocks 151 and 152, annular piezoelectric elements 153, 154, 155, 156, electrodes 157, 158, 159, and a bolt 160 connecting the blocks 151 and 152 with the piezoelectric elements and the electrodes interposed therebetween. The piezoelectric elements have a positive polarity on their half side and a negative polarity on the other half side. When high voltage is applied to the left side surface of the oscillator in the drawing, each positive part "a" expands and each negative part "b" contracts. On the other hand, when high voltage is applied to the right side surface of the oscillator, each positive part "a" contracts and each negative part "b" expands. The electrode 158 and the pair of the metal blocks 151, 152 are grounded, and the electrodes 157, 159 are applied with sine wave voltage shown in FIG. 16. The electrical phases of voltage applied to the electrode 157, 159 are displaced with each other as shown in FIG. 16.
According to the Langevin type oscillator, a progressive wave is generated at the front end of the block 151 as shown by an arrow 162, thus rotating a cylindrical spool 161 in the direction of the arrow 162. By using this phenomenon, a film can be wound on the spool 161.
In addition to U.S. Pat. No. 5,122,,700, U.S. Pat. Nos. 4,697,117, 4,812,697 and 4,652,786 also disclose such an oscillator which rotates a rotor or a spool by a progressive wave generated on a block.
Many researches have been made on the progressive-wave based oscillator since it is very useful for rotating a rotor or a spool in place of a conventional motor widely used. However, an oscillator including piezoelectric elements is not restricted to the progressive-wave based oscillator. Further, functions other than "rotation" of a rotor, a spool, etc. may be required for the oscillator. The inventors of the present application have noted the above points and attained an idea to use bending vibrations.
Vertical and torsional vibrations have been used in the progressive-wave based Langevin type oscillator. In contrast, the inventors use vertical and bending vibrations. The most basic structure of this type of oscillator is disclosed in Japanese Laid-Open Patent Publication No. 64-74072. The basic structure is shown in FIGS. 13(a) to 13(c). FIG. 13(a) is an exploded perspective view of the vibration type driving device in the prior art, and FIG. 13(b) is a sectional view in a central plane of the device in the assembled condition. The driving device includes a metal rod 102 which is provided at both ends thereof with threaded portions 102a, 102b. The threaded portions 102a, 102b are adapted to secure a pair of metal blocks 92, 104 to the respective ends of the rod 102. The metal blocks 92, 104 have respective threaded holes 92a, 104a into which the threaded portions 102a, 102b are engaged to fasten the pair of metal blocks 92, 104 to the respective ends of the metal rod 102.
The device further includes an annular conductor plate 94, an annular piezoelectric element 96, a pair of semicircular conductor plates 98a, 98b and another annular piezoelectric element 100. The conductor plate 94, the piezoelectric element 96, the pair of conductor plates 98a, 98b and the piezoelectric element 100 are interposed between the pair of metal blocks 92 and 104.
As shown in FIG. 13(b), the conductor plate 94 is in contact with the threaded portion 102a of the rod 102, so that the metal block 92, the metal rod 102 and the metal block 104 are all electrically connected to the conductor plate 94. When in use, the conductor plate 94 is grounded so as to ground the metal block 92, the metal rod 102 and the metal block 104. The semicircular conductor plates 98a, 98b are insulated from the rod 102. The conductor plates 98a, 98b are also insulated from each other. The semicircular conductor plates 98a, 98b are adapted to cover corresponding parts of the piezoelectric elements 96, 100. When in use, high frequency voltage of several tens KHz is applied to either one of the semicircular conductor plates 98a, 98b.
FIGS. 14(a) to 14(c) show the phenomenon generated by application of high frequency voltage to one of the semicircular conductor plates, in the illustrated example, 98a in a rather exaggerated manner. When high frequency voltage is applied to the semicircular conductor plate 98a, the thickness of parts of the piezoelectric elements 96, 100 covered with the conductor plate 98a is increased and decreased, causing vertical vibrations of the assembly DV of the rod 102 and the pair of blocks 92, 104, as shown in FIGS. 14(a) and 14(c). As no voltage is applied to the other semicircular conductor plate 98b, there is no variation in thickness of parts of the piezoelectric elements 96, 100 covered with the conductor plate 98b. Consequently, the assembly DV carries out bending vibrations, as shown in FIGS. 14(b) and 14(d). Here, the length of the assembly and the frequency of the applied voltage can be so set that the end surface 91 of the assembly DV may define the resonance point of the vertical vibrations and simultaneously the resonance point of the bending vibrations. Thus, the end surface 91 of the assembly DV can move in an elliptical path, as shown by arrows RL, RR in FIG. 14(d).
When an article to be driven by this device is placed on the end surface 91 of the assembly DV, the movement of the end surface in the direction of the arrow RL causes the article to be moved in the direction of an arrow X1. During movement of the end surface 91 of the assembly DV in the direction of the arrow RR, contact pressure between the end surface 91 of the assembly DV and the article is reduced, due to inertia of the article to be driven as well as high frequency of the elliptical vibrations defined by the arrows RL, RR. This results in reduced force generated for moving the article to the left in the drawing, though the end surface 91 is moved in the direction of the arrow RR. Thus, the article can be driven in the direction of the arrow X1 in the drawing.
When high frequency voltage is applied to the other semicircular conductor plate 98b, the article to be driven is moved in the opposite direction (the direction of an arrow X2 in FIG. 13). In case driving operation is necessary only in one direction, either one of the semicircular conductor plates 98a, 98b may be formed of an insulating material.
The Langevin type oscillator described above avoids using a progressive wave, permitting an article to be moved linearly. Further, displacement or adjustment of the electrical phase of applied voltage is not required, thus drastically simplifying the driving circuit.